In general, such ducts are known and are built up of elements suitable for being interconnected, each having a reduced-diameter male endpiece at one end, and an enlarged-diameter female endpiece at its other end.
In domestic installations, where combustion gases are to be discharged from a cooker, connection between two adjacent duct elements is generally the result of a friction assembly, with successive elements merely being fitted one within another. It is also known to place collars along the length of pipes assembled together in this way for the purpose of fixing such pipes to an adjacent wall, and such collars can optionally serve to reinforce the connection between two successive elements.
Large-diameter chimneys for industrial installations are generally made either of concrete or of steel, with the steel elements being united via flanges that are welded on in the factory and being transported directly to the site.
For installations of medium size, the present practice is to use modular ducts made up of tubes that are connected together end to end. Sealing means may be provided between two tubes, but unfortunately such sealing is insufficient against condensed flue gases, particularly when the duct is used with a counter slope. In addition, they are unsuitable for being cut to length, for example so that the collars correspond to joints between facing stones.